This invention pertains to a sintered self-reinforced silicon nitride ceramic body and to compositions used in preparing the same.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance, and high electrical resistivity. In addition, silicon nitride ceramics resist chemical attack, particularly oxidation. Because of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Typically, densification of silicon nitride requires the presence of densification aids, such as oxides of magnesium, yttrium, aluminum, cerium, silicon, and zirconium. A powder mixture comprising silicon nitride and one or more of such densification aids is usually prepared and heated under conditions described hereinafter. The densification aids form a liquid phase into which .alpha.-silicon nitride dissolves and from which it precipitates as .beta.-silicon nitride.
The final density of a ceramic body depends largely on the density of the body before heat is applied. This is often referred to as "green density," and the body is referred to as "greenware." The most common method of forming greenware is dry pressing. However, a problem with dry pressing is that it does not favor forming large and complex shaped bodies. In addition, forming greenware by dry pressing typically results in low density parts with non-uniform distributions of porosity. The non-uniformity generally relates to a higher density at the edge of a ceramic part than in the center. Further, binders, used to process the bodies into their near net shapes, usually have to be removed. De-bindering is a long and difficult process that often leads to development of internal cracks within the body.
An alternative method of forming ceramic greenware is by colloidal processing, such as slip-casting. An advantage of colloidal processing is that large, complex shaped, high density greenware can be produced without the use of binders. A second advantage is that aqueous carrier media may be used, eliminating the need for more expensive or potentially environmentally hazardous processing conditions. Notwithstanding these advantages, colloidal processing has one major disadvantage in that as the number of components in a formulation increases, it becomes more difficult to find common colloidal processing conditions. This is because every component has different surface characteristics, and these characteristics determine the conditions in which colloidal processing will work. Thus, for many multi-component ceramic compositions, it is difficult to form ceramic greenware by colloidal processing.
Typically, in order to obtain substantially full densification of the aforementioned powder mixture or greenware, one of four general methods is used: hot pressing (HP), hot isostatic pressing (HIP), pressure-less sintering, or low pressure gas sintering. Densification of silicon nitride alone normally does not go to completion in the absence of high pressure. For example, the density of the silicon nitride ceramic body might only reach 80 or 90 percent of its theoretical value. Whereas a density of at least 98 percent is required to achieve a ceramic having excellent mechanical and physical properties, such as high fracture strength and high fracture toughness. Further, at high temperatures and low pressures, silicon nitride decomposes into elemental silicon and nitrogen. Thus, the commercial need for fully densified silicon nitride ceramics, having excellent fracture strength and fracture toughness, is currently met predominantly by using HP or HIP to densify combinations of silicon nitride and densification aids.
Disadvantageously, however, the HP and HIP methods require complicated high pressure equipment and typically yield only a ceramic having a simple shape. In order to obtain a more complicated net shape, the densified HP or HIP ceramic typically must be subjected to post-densification procedures such as diamond grinding. Although more complicated shapes may be obtained by pressureless or low pressure gas sintering, these methods typically present difficulties in obtaining ceramic bodies of substantially full density having high fracture strength and toughness.